THE GANGDESE RETROARC FOLD-THRUST BELT REVEALED

The Cretaceous-early Tertiary Gangdese batholith has long been attributed to northward oceanic subduction beneath the southern Lhasa terrane continental margin, with convergence rates of >10 cm/yr. While shortening of Gangdese age has been documented, a retroarc fold-thrust belt typical of other Andean-style margins has not been recognized. We suggest that previously and newly mapped Late Cretaceous contractional structures in the Lhasa area are components of a major N-directed and propagating fold-thrust belt. Here, thrust relationships with upper Paleozoic strata in the hanging wall are exposed over a N-S distance of ~100 km. ~10 km south of Lhasa, upper Paleozoic metasedimentary rocks are thrust northward over Triassic to Upper Cretaceous (Takena Formation) strata. In the Penbo area (~40 km north of Lhasa), the Takena Formation occurs in the footwall of a N-dipping stack of Carboniferous-Cretaceous thrust sheets with Carboniferous strata in the hanging wall of the structurally highest thrust. All involved units are strongly deformed by N-directed overturned folds and structural analysis of thrust fault zones, along with fault branching relationships, indicate top-to-the-north sense-of-shear. North of Penbo, Carboniferous strata are exposed over a N-S distance of ~60 km. Locally mapped Cretaceous strata may be exposed in structural windows to a Carboniferous thrust sheet. North of Damxung, in the Nyainqentanglha range, a newly discovered S-dipping thrust fault juxtaposes upper Paleozoic strata northward over Cretaceous and older metasedimentary rocks. All of the identified upper Paleozoic over younger thrust relationships are Late Cretaceous as they cut Upper Cretaceous strata, are intruded by Gangdese-age granites, and are unconformably overlain by 68 to 48 Ma Linzizong volcanic rocks. We suggest that they represent a single N-directed megathrust with >100 km of slip and which roots south of Lhasa, serves as a roof thrust to an antiformal stack near Penbo, and resurfaces in the Nyainqentanglha. The presence of a major N-directed thrust system resolves the dilemma of a "missing" Gangdese retroarc fold-thrust belt and is consistent with our recent documentation of northward-younging, southerly-derived nonmarine clastic deposition in the Lhasa terrane during Late Cretaceous-early Tertiary time.